In general Cabal therapist is much slower that Cabal therapy, you need 3 turns to do what cabal therapy can do in one turn. And he can't be cast freely from the graveyard. That's why most deck that currently play cabal therapy will probably no use Therapist.
But I see two good points to Therapist
As long as we have other creature we can do the effect every turn. It can be use to setup some soft lock, similar to what Liliana of the Veil can do.
Some decks like TPS cast Cabal Therapy from their hand the turn they want to combo off. With Therapist you can play it some turn before and then save 1 black mana and have an uncounterable therapy effect the turn you want to combo off.